More than 528,000 Americans have signed up for Affordable Care Act coverage on the federal exchange in the first six weeks of the special enrollment period President Joe Biden opened for the uninsured. That’s more than double the number that turned to the exchange, healthcare.gov, in the same period over the past two years, when enrollment was limited to those losing job-based policies and experiencing other major life events, such as divorce, according to data released Wednesday by the Department of Health and Human Services.